Exercise for meditation sitting

I couldn't find this exact question asked before. What kind of exercise is recommended for better physical comfort and "endurance"/"stamina" when sitting in meditation; apart from the sitting itself? I usually sit in the half vajra position, which is quite comfortable and stable, but find it a bit tiring when I sit for many hours. Any good exercises in "the long run"? Any good stretching or other exercises between sessions when sitting whole days?